Reflyer has a back end server that powers the app. Users are not asked to register or identify themselves, so any data we collect there is totally anonymous. That said, we don't collect much, basically just for analytics and to remember any actions you take, so we can understand how people are using the app and record actions like shares and "remind me"'s.
We also send 100% anonymous usage data (without even a user's reflyer userId) to a couple industry-standard analytics tracking firms, so we can understand how our users are using reflyer.
(up to date as of Sept 1, 2017)